While images abound of evil black knights rampaging the countryside, these tales are far from the truth. A black knight is a knight not sponsored or employed by a local lord for the defense of his lands. Therefore, the black knight does not have at his disposal the resources needed to keep his armor polished and in good repair. To save face, these knights painted their armor black, to hide any wear and tear that the armor may have endured.

The black knight knelt before the Duke, hoping to end his long bout of unemployment.

The Urban Dictionary T-Shirt

Usually a young male, although shows acts of chivilary and kindness, usually has little regard for rules and regulation, and adopts a roguish attitude to his ventures. Often the antagonist of a White Knight.

"That guy just rescued 30 people from a bank siege! After he beat up the criminals, he found out the bank manager was the one who organised the holdup, so he kicked his ass and stole his wallet and car! That guy was such a Black Knight"

The Urban Dictionary T-Shirt

In contrast to a White Knight, a Black Knight is any one person who- rather than frantically defending women who are being badgered by men on the internet- is always attacking women who are badgering men.

MarcoMarcoXD: who likes the new mario game
MinerChick96: ur a fucking loser dude lol
MarcoMarcoXD: :(
xXBlackKnightXx: Listen here, fucker. Marco didn't do shit to deserve that kind of talk, stop weaseling your way into other people's business and fuck off. Nobody gives a shit about what you have to say and he certainly doesn't fucking need to hear it either, so crawl back to where you came from and stay there, cunt.
MinerChick96 has logged off.
MarcoMarcoXD: thx blackknight
Admin has kicked user xXBlackKnightXx for reason: "stop your black knight bullshit, kid."

The Urban Dictionary T-Shirt

In Korean 흑기사 ("Black Knight"), this word would mean someone (generally a guy) who volunteers to take a drink for another person (generally a girl). For a man it’s 흑기사 (heuk-gi-sa, “dark knight”) or 흑장미 (heuk-jang-mi, “black rose”) for women. One can ask someone to be their black knight/rose, or someone can offer to do it, but once you’ve done it, you’re theirs for the rest of the night. If you ask someone and they refuse, then you have to take two drinks.

While playing a drinking game, a girl has had too much to drink but has to take a shot. She can then call out for a Black Knight or Dark Knight and a generous guy may take the drink for her.